#----------------------------------------------------------------------
# Image filtering
# Matrix is a 3x3 grid of multipliers for the neighboring pixels
# around each pixel from the source image.  The 3x3 grid is represented
# by a list of lists: [[_,_,_],[_,_,_],[_,_,_]].
# See page 292 in the Companion.
# Copyright (C) March 27, 2017 -- Dr. William T. Verts
#----------------------------------------------------------------------

def Filter (Canvas, Matrix, Scale=1, Offset=0):

#   NewCanvas = makeEmptyPicture(getWidth(Canvas),getHeight(Canvas))
    NewCanvas = duplicatePicture(Canvas)
    
    for Y in range(getHeight(Canvas)):     # Scan all rows
        for X in range(getWidth(Canvas)):  # Scan all columns
            TotalR = 0
            TotalG = 0
            TotalB = 0
            for YY in range(max(Y-1,0),min(Y+2,getHeight(Canvas))):      # Scan Y neighbors, clip to top & bottom
                for XX in range(max(X-1,0),min(X+2,getWidth(Canvas))):   # Scan X neighbors, clip to left & right
                    PX = getPixel(Canvas,XX,YY)
                    R = getRed(PX)
                    G = getGreen(PX)
                    B = getBlue(PX)
                    Yindex = YY - Y + 1    # Convert Canvas-relative coordinates back
                    Xindex = XX - X + 1    # into Matrix-relative coordinates
                    Multiplier = Matrix[Yindex][Xindex]
                    TotalR = TotalR + R * Multiplier
                    TotalG = TotalG + G * Multiplier
                    TotalB = TotalB + B * Multiplier
            TotalR = TotalR / Scale + Offset
            TotalG = TotalG / Scale + Offset
            TotalB = TotalB / Scale + Offset
            NewColor = makeColor(TotalR,TotalG,TotalB)
            PX = getPixel(NewCanvas,X,Y)
            setColor(PX,NewColor)
        repaint(NewCanvas)
    return NewCanvas

#----------------------------------------------------------------------
            
def Main():
    Filename = pickAFile()
    Canvas = makePicture(Filename)
    show(Canvas)
#   Uncomment the line for the desired filtering function below:
#    Canvas = Filter(Canvas, [[-1, 0,-1],[ 0, 7, 0],[-1, 0,-1]],  3,   0)  # Focus
#    Canvas = Filter(Canvas, [[-1,-1,-1],[-1, 8,-1],[-1,-1,-1]],  1,   0)  # Edge Detect
#    Canvas = Filter(Canvas, [[-1,-1,-1],[-1, 8,-1],[-1,-1,-1]], -1, 255)  # Edge Detect and Negate
#    Canvas = Filter(Canvas, [[ 1, 0, 0],[ 0, 0, 0],[ 0, 0,-1]],  1, 128)  # Emboss UL          
#    Canvas = Filter(Canvas, [[-1, 0, 0],[ 0, 0, 0],[ 0, 0, 1]],  1, 128)  # Emboss LR          
#    Canvas = Filter(Canvas, [[ 0, 0, 1],[ 0, 0, 0],[-1, 0, 0]],  1, 128)  # Emboss UR          
#    Canvas = Filter(Canvas, [[ 0, 0,-1],[ 0, 0, 0],[ 1, 0, 0]],  1, 128)  # Emboss LL          
#    Canvas = Filter(Canvas, [[ 1, 1, 1],[ 1, 1, 1],[ 1, 1, 1]],  9,   0)  # Blur        
    repaint(Canvas)
    return